Such an instrument is described in the article "SEVIRI, the new imager for Meteosat second generation", by F. Pasternak et al, which appeared in "Proceedings of 1993 IGARSS Symposium, Volume III". This instrument includes an optical detection assembly situated at the location of the focal plane of the telescope, taking images of the Earth in various infrared or visible wavelength channels. This optical assembly is associated with a cooling system intended to keep the detectors at a temperature which is as low as possible; this reduces the detected noise. The telescope is a Ritchey-Chretien telescope including a primary concave mirror receiving an incoming image which is reflected back by a plane scanning mirror, and a convex secondary mirror disposed coaxially with the primary mirror and reflecting back the image reflected by the latter towards the focal plane of the telescope through a central aperture formed in the primary mirror. A disadvantage of this telescope is the relatively great height between the base of the scanning unit and the focal plane of the telescope (at least 1500 mm). This bulkiness reduces the space available for the system for cooling the detectors, and thus limits the possibilities for cooling.
Other types of telescope have likewise been proposed, in particular Gregory telescopes. However, their bulkiness likewise causes difficulties, and they frequently require relatively complex screen structures to reduce the interferences caused by the extraneous light.
As described in the aforementioned publication, the telescope is, in general, associated with a calibration source for the infrared channels. In the calibration phase, this source covers the whole of the entrance aperture of the telescope, and must therefore be of relatively large dimensions; this presents problems of bulkiness and of temperature regulation.